Supporting mobility of users is a basic and very important feature of mobile radio systems [3-5]. Users of such a system expect that the connectivity is maintained when moving from the coverage area of one base station into the coverage area of a second base station.
Modern mobile radio systems such as LTE are based on a frequency reuse factor of one, meaning that each base station uses the whole frequency band. Although it provides high spectral efficiency, the reuse-one deployment will potentially cause significant inter-cell interference (ICI), especially on cell borders. To guarantee a very high handover success-rate becomes challenging, as the successful handover procedure requires good radio link quality to convey necessary control signalling messages between the mobile and the serving/target base stations. At the initial stage of a handover process, the mobile station should be able to receive the Handover-Command from the serving base station in order to execute the handover. During this stage, the target base station often acts as the strongest interferer, causing significant ICI. This can prevent the successful transmission of any signalling message, and in the worst case lead to drop of the connection of the mobile station to the serving base station (resulting in the so-called handover failure). At the succeeding handover stage after the handover has been executed, the mobile station is synchronizing to the target base station by exchanging control signalling with the target. The poor radio link in the target cell induced by ICI can also make the handover a failure.
Normal Inter-Cell Interference Coordination (ICIC) in LTE Release 8/9, e.g. Fractional Frequency Reuse, Soft Frequency Reuse, can be used to mitigate interference in order to support handover. However, it does not improve the performance of the control channels which are especially important for a reliable communication between mobile and base station.
Heterogeneous network (HetNet) deployment of Long Term Evolution (LTE)-Advanced has recently attracted lots of research activities [1, 2]. The main idea of HetNets is to overlay low-power and low-cost base-station (BS, called eNB in LTE) with the conventional macro cellular networks. By deploying such low-power low-complexity eNBs on indoor coverage holes or cell edges, coverage extension can be achieved in a cost-effective way. They can also be deployed on traffic-demanding hotspots to boost local capacity by frequency reuse [2].
Among the low-power eNBs in a HetNet, pico BSs are powerful equipments with the only difference of having lower transmit power than traditional macro cells. They are typically deployed by operators and operated in an open-access mode [1]. However, the promising benefits brought by the macro-pico deployment come along with the new challenges for system design, one of which is in the handover (HO) process.
In conventional homogeneous network, the HO boundary coincides with the cell border induced by the downlink (DL) transmit power of BSs. In a HetNet, however, this HO boundary will lead to the case where the macro BSs become resource constrained while the pico BSs serve very few users, due to the much stronger transmit power of macro BSs. Hence, the HO decision should be made jointly considering the load balance, user mobility and the signal strength [6]. Recently, range expansion (RE) techniques have been considered in 3GPP to offload macro UEs to pico cells by adding a positive bias to the DL signal strength of pico BSs during the cell selection [8, 9]. However, the pico UEs in the expanded-range potentially suffer from the degraded signal-to-interference-plus-noise radio (SINR) in the DL since they are not connected to cells that provide the highest signal strength.
In order to address the more complicated interference scenarios in heterogeneous networks (HetNets), enhanced ICIC (eICIC) techniques have recently been developed for Release 10, which can be classified into the following three categories according to [19]:                Time-domain techniques        Frequency-domain techniques        Power control techniques        
By using these techniques, the interference in the control channel can be alleviated. For example, in the time-domain eICIC approaches, the macro nodes are periodically muted at certain subframes to configure so-called Multicast-Broadcast Single Frequency Network (MBSFN) subframes or Almost Blank Subframes (ABS), which will be called “protected subframes” in the following. In those subframes, there is no data transmission in the macro cells. The control channel transmission is also absent or light-loaded. Then the victim pico UE (which is often in the expanded range) can be scheduled in subframes corresponding to protected subframes of the macro nodes, which significantly mitigates the interference of macro to pico. However, in order for the victim pico UE to enjoy a macro-interference-free environment, all the macro nodes in the network should configure the same patterns of protected subframe, and the network synchronization should be perfect. In the following, this eICIC method will be referred to as “Static-ICIC”. As will be shown later, this Static-ICIC has only limited ability to improve the handover performance, because only the pico UEs are protected against the interference coming from macro cells during a handover. When the macro UE wants to handover to the neighboring cell or the pico UE performs handover to the neighboring pico cell, this Static-ICIC will not help to reduce the interference.
The ABS can be configured not only at the macro nodes, but also at the pico nodes. In [7], the authors proposed a Mobility-Based-ICIC to enhance the handover performance. Basically, the Mobility-Based-ICIC is a static approach and it is targeted at reducing the failure rate for macro-to-pico handovers for high-speed UEs. For this approach, both pico and macro nodes reserve certain static pattern of protected subframes, and it depends on both the handover types and the UE mobility state (e.g., low-mobility or high-mobility) to utilize the protected subframes. If the handover occurs from pico to macro, the pico nodes can schedule the UE in the protected subframes of macro nodes, just as the previous-mentioned Static-ICIC. On the other hand, if the handover occurs from macro to pico, the macro node can schedule their high-mobility (>=60 km/h) in the protected subframes of pico nodes without co-channel interference from pico nodes. Since low-mobility macro UEs are less vulnerable to handover failures, they will be scheduled in the normal subframes. As can be seen from the above description, the Mobility-Based-ICIC has the potential to improve the inter-layer (macro-pico, pico-macro) handovers, but lacks the ability to handle the intra-layer (macro-macro, pico-pico) handovers, because there is no intra-layer interference coordination mechanism for this method. Besides, the detection of the UE mobility requires additional computational resources and complexity.